Titanium alloys based on the intermetallic compound Ti3Al (and also known as alpha-two materials) have been the subject of interest and investigation for a number of years. These materials offer the promise of good high temperature properties in combination with low density and useful oxidation resistance.
Heretofore however, these alloys have not found application because of limited low temperature ductility. While certain of these alloys can be hot worked at temperatures near and above their beta transus temperatures (typically 1950.degree.-2150.degree. F.), the room temperature ductility of these materials has been on the order of 3-7% maximum tensile elongation with 1-3% being typical. Materials with such low ductilities are not desirable engineering materials because it is difficult to fabricate them into useful shapes except at high temperatures and their utilization at low temperatures can be a problem because of the potential for handling damage. Cracks formed by mishandling during production and assembly could propagate during service leading to failure.
A complete understanding of the invention requires knowledge of the phase relationships in these alloys. Two phases can occur, the alpha-two phase has an ordered hexagonal close packed crystal structure while the beta phase has body centered cubic structure. All materials which are useful in conjunction with the invention are 100% beta above a certain temperature known as the beta transus. When cooled below this temperature they transform wholly or partially to alpha-two. Some amount of residual beta is desired since it appears to enhance ductility, however the invention is applicable to material which is entirely alpha-two at room temperature.
U.S. Pat. Nos. 4,292,077 and 4,716,020 which share some common inventors with the present invention and which are assigned to the same assignee describe two of the most successful alpha-two type alloys. These alloys have the best combination of properties heretofore obtained in this alloy field. These properties are obtained by careful compositional control. U.S. Pat. No. 4,292,077 discloses vanadium additions to titanium-aluminum-niobium alpha-two type alloys for increased ductility where vanadium generally substitutes for titanium. Tables 2 and 4 in this patent show room temperature ductility values for the invention alloys with a maximum ductility of 4% being shown in Table 2 and a maximum of 1.3% being shown in Table 4. U.S. Pat. No. 4,716,020 adds molybdenum to the alloys of 4,292,077, the maximum low temperature elongation disclosed in this patent appears to be 2.2% as shown in Table 1, although a number 2.5% is mentioned in column 3 at line 38.
These two patents suggest similar processing techniques, specifically "solutionizing or forging should be conducted above the beta transus followed by aging between 700.degree.-900.degree. C. for 2-24 hours (U.S. Pat. No. 4,716,020 column 5, lines 20-25).
As used herein, tensile elongation is determined using a 0.75 inch gauge length specimen. All compositions are listed as weight percents unless otherwise noted.